1. Field of the Invention
The invention relates to a mixing ratio detecting apparatus for detecting the mixing ratio of a component to be detected to a mixture solution containing the component to be detected. In particular, the invention relates to a mixing ratio detecting apparatus for detecting the mixing ratio of methanol and water to be used in a direct methanol fuel cell or the like. In other words, the invention relates to a so-called methanol aqueous solution concentration sensor.
2. Description of the Related Art
Fuel cells are devices for generating electric energy from hydrogen and oxygen, and are capable of providing high generation efficiency. The fuel cells chiefly has the following characteristics: high generation efficiency is expected even in smaller scales because of the direct generation mode without thermal- or kinetic-energy processes as in conventional generation modes; and excellent environmental friendliness is obtained from low emission of nitrogen compounds as well as reduced noise and vibrations. Since the fuel cells can thus use the chemical energy of the fuel effectively and have the environment-friendly characteristics, they are expected as energy supply systems to bear the 21st century. For various applications ranging from large-scale power generation to small-scale generation, such as space technologies, automobiles, and portable devices, the fuel cells are attracting attention as promising novel generation systems. Technological development toward practical use has thus been made in earnest.
Above all, solid polymer type fuel cells are characterized in lower operating temperatures and higher output densities as compared to the other types of fuel cells. Among various forms of solid polymer type fuel cells, a direct methanol fuel cell (DMFC) has recently been gaining attention in particular. The DMFC is one in which a methanol aqueous solution, the fuel, is supplied directly to the anode without any modification so that electricity is generated through the electrochemical reaction between the methanol aqueous solution and oxygen. In this electrochemical reaction, carbon dioxide and generated water are emitted from the anode and the cathode as reaction products, respectively. As compared to hydrogen, the methanol aqueous solution provides higher energy per unit volume, is well-suited to storage, and has low risk of explosion or the like. Applications such as the power sources of automobiles and cellular phones are thus expected.
When the methanol aqueous solution to be supplied to the anode of the DMFC is too high in concentration, there can occur the problem that degradation of the solid polymer film inside the DMFC is promoted with a drop in reliability. When the concentration is too low, it is impossible to obtain a sufficient output from the DMFC. The concentration is thus preferably adjusted within the range of 0.5 and 4 mol/L, and desirably 0.8 and 1.5 mol/L. It has been found that the range of concentrations can be narrowed to stabilize the operation of the DMFC.
Now, take the case of a system having a DMFC. For the sake of operating the DMFC for a long period and reducing the size and weight of the system as well, the system is typically provided with a tank for containing methanol having a high concentration of 20 mol/L or above. Here, the methanol must be thinned and adjusted in concentration before supplied to the anode of the DMFC. Then, a methanol aqueous solution concentration sensor of high precision and small size has been sought after in order to adjust the methanol aqueous solution to a concentration of 0.8 to 1.5 mol/L inside the system. Researches have been made various types of sensors including optical type (see Japanese Patent Laid-Open Publication No. 2001-124695), supersonic type, and specific gravity type. Nevertheless, the methanol aqueous solution concentration sensor to be implemented on a DMFC system is yet to be developed since it requires high precision, small size, light weight, low cost, and low power consumption.
The foregoing optical type concentration sensors have had the possibility of a drop in measurement precision if the aqueous solution contains air bubbles, contaminations, etc. Besides, full-time operation of the detecting apparatus has produced the problem of large power consumption for measurement.